1. Field of the Invention
This invention relates generally to video tape apparatus, and more particularly is directed to a system for controlling both the phase and speed of the rotary head or heads of such apparatus by means, such as a brake, but with the speed signal being derived so that, if, for example, the information was recorded with the tape running at a speed different from the customary speed, the tape can be run at such different speed during reproducing or playback without adversely affecting the phase of the rotating head.
2. The Prior Art
Video tape apparatus normally includes a supply of tape on which information signals may have been recorded previously or may be recorded by the apparatus itself. The tape is wrapped in a helical path at least partially around a drum with which one or more rotary heads or transducers are associated to move in a circular path in a plane that intersects the longitudinal direction of the helically wrapped tape. As the tape is advanced longitudinally by a capstan that normally operates at a fixed speed, video or information signals are recorded by the rotary head or heads in successive parallel tracks that extend obliquely across the surface of the tape, and control signals are recorded by a fixed head or transducer along one edge of the tape so as to be in predetermined positional relation to the oblique tracks. Since the information signals and the control signals are recorded while the tape is moving, the distance between the positions of the successive control signals on the tape are determined by the speed at which the tape is moving. The control signals to be recorded may be derived from pulse signals generated in a magnetic pick-up device located adjacent the rotary shaft of the rotary head or heads to thereby correlate the speed and phase angle of the rotating head or heads with the speed and linear position of the tape during recording.
During playback of the recorded tape, the same type of pulse generator or rotational speed detector produces pulse signals having a repetition rate that is a function of the speed of rotation of the magnetic head or heads. These pulse signals also indicate a specific angular, or phase position of each rotating head at the time each pulse signal is generated. The pulse signals are compared in phase with the control signals reproduced from the tape to provide a phase responsive signal which corresponds to the relative position, or phase, of the rotating head and the linear position of the tape. The phase responsive signal thus derived is employed to control or regulate a brake to slow down the rotation of the reproducing head or heads to a greater or lesser degree as necessary to cause each head to start to scan a slant area of the tape at a point that corresponds to the start of one of the oblique tracks in which video or other information signals are recorded on the tape.
The playback or reproducing apparatus further includes means responsive to the pulse signals from the rotational speed detector to produce a signal responsive to the speed of rotation of the head or heads so that the slant area traversed by each head will not only start at the same point as the beginning of a respective one of the oblique tracks in which video signals were previously recorded, but will continue directly along such oblique tracks. The speed responsive signal, as well as the phase responsive signal, is applied to the brake to affect its operation.
With the above described speed and phase control or servo system of the prior art, for example, as disclosed in U.S. Pat. No. 3,495,152, or in U.S. Pat. No. 3,836,756, if the tape was originally recorded with a rotary head rotating at the standard speed of, for example, 30 r.p.s., then, during correct reproduction of such tape, the reproduced control signals and the pulse signals have repetition rates of 30Hz with a predetermined phase difference therebetween. So long as the pulse signals have a repetition rate of 30Hz and there is the predetermined phase difference between the control signals and the pulse signals, the voltage of the speed responsive signal and the voltage of the phase responsive signal combine to produce a brake control voltage or signal by which the phase and speed of rotation are maintained correct. If the rotating speed of the head is, for example, momentarily reduced below the standard speed, the voltages of the speed responsive signal and phase responsive signal are suitably changed to vary the brake control voltage or signal so as to restore the correct phase and speed relationships.
However, if the tape was originally recorded while being advanced at an incorrect speed, for example, at a speed slightly less than the standard speed, the spacing between the recorded control signals is correspondingly reduced, so that, during reproduction with the tape being advanced at the standard speed, such control signals have an increased repetition rate, for example, of 30.1 Hz., and in order to correctly scan the resulting oblique record tracks, the rotary head or heads must rotate at a relatively increased speed at which the pulse generator or rotational speed detector produces the pulse signals with a repetition rate of 30.1 Hz. Such increased repetition rate of the pulse signals causes a decrease in the speed responsive voltage so that, in the absence of any change in the voltage of the phase responsive signal, the brake control voltage or signal would be varied in the direction to increase the braking force and thereby reduce the rotational speed of the rotary head. Thus, the phase responsive signal compensates for the decrease of the voltage of the speed responsive signal, and this is achieved at the expense of changing the phase difference between the control signals and the pulse signals so that the rotated head no longer accurately scans the oblique tracks on the magnetic tape.